For more information about this option, see the following section(s) in the ANSYS Structural Analysis Guide:
Option
• Section 8.5.2.3.3: Maximum Number of Equilibrium Iter-ations
Specify analysis termination criteria [NCNV]
• Section 8.10.2.5: Using the Arc-Length Method
• Chapter 2, “Loading” in the ANSYS Basic Analysis Guide Control activation and termination of the
arc-length method [ARCLEN, ARCTRM]
2.3.3. Set Additional Solution Options
This section discusses additional options that you can set for the solution. These options do not appear on the Solution Controls dialog box because they are used very infrequently, and their default settings rarely need to be changed. ANSYS menu paths are provided in this section to help you access these options for those cases in which you choose to override the ANSYS-assigned defaults.
Many of the options that appear in this section are nonlinear options, and are described further in Chapter 8,
“Nonlinear Structural Analysis”.
2.3.3.1. Stress Stiffening Effects
Some elements, including those in the 18x family of elements, include stress stiffening effects regardless of the SSTIF command setting. To determine whether an element includes stress stiffening, refer to the appropriate element description in the ANSYS Elements Reference.
By default, stress stiffening effects are ON when NLGEOM is ON. Specific situations in which you can turn OFF stress stiffening effects include:
Section 2.3: Performing a Static Analysis
• Stress stiffening is relevant only in nonlinear analyses. If you are performing a linear analysis [NLGEOM,OFF], you can turn stress stiffening OFF.
• Prior to the analysis, you know that the structure is not likely to fail because of buckling (bifurcation, snap through).
Including stress stiffness terms, in general, accelerates nonlinear convergence characteristics. Keeping in mind the points listed above, you may choose to turn stress stiffening OFF for specific problems in which convergence difficulties are seen; for example, local failures.
Command(s): SSTIF
GUI: Main Menu> Solution> Unabridged Menu> Analysis Options
2.3.3.2. Newton-Raphson Option
Use this analysis option only in a nonlinear analysis. This option specifies how often the tangent matrix is updated during solution. You can specify one of these values:
• Program-chosen (default)
• Full
• Modified
• Initial stiffness
• Full with unsymmetric matrix Command(s): NROPT
GUI: Main Menu> Solution> Unabridged Menu> Analysis Options
2.3.3.3. Prestress Effects Calculation
Use this analysis option to perform a prestressed analysis on the same model, such as a prestressed modal ana-lysis. The default is OFF.
Note — The stress stiffening effects and the prestress effect calculation both control the generation of the stress stiffness matrix, and therefore should not be used together in an analysis. If both are specified, the last option specified will override the previous setting.
Command(s): PSTRES
GUI: Main Menu> Solution> Unabridged Menu> Analysis Options
2.3.3.4. Mass Matrix Formulation
Use this analysis option if you plan to apply inertial loads on the structure (such as gravity and spinning loads).
You can specify one of these values:
• Default (depends on element type)
• Lumped mass approximation
Note — For a static analysis, the mass matrix formulation you use does not significantly affect the solution accuracy (assuming that the mesh is fine enough). However, if you want to do a prestressed dynamic analysis on the same model, the choice of mass matrix formulation may be important; see the appropriate dynamic analysis section for recommendations.
Command(s): LUMPM
GUI: Main Menu> Solution> Unabridged Menu> Analysis Options
2.3.3.5. Reference Temperature
This load step option is used for thermal strain calculations. Reference temperature can be made material-de-pendent with the MP,REFT command.
Command(s): TREF
GUI: Main Menu> Solution> Load Step Opts> Other> Reference Temp
2.3.3.6. Mode Number
This load step option is used for axisymmetric harmonic elements.
Command(s): MODE
GUI: Main Menu> Solution> Load Step Opts> Other> For Harmonic Ele
2.3.3.7. Creep Criteria
This nonlinear load step option specifies the creep criterion for automatic time stepping.
Command(s): CRPLIM
GUI: Main Menu> Solution> Unabridged Menu> Load Step Opts> Nonlinear> Creep Criterion
2.3.3.8. Printed Output
Use this load step option to include any results data on the output file (Jobname.OUT).
Command(s): OUTPR
GUI: Main Menu> Solution> Unabridged Menu> Load Step Opts> Output Ctrls> Solu Printout Caution: Proper use of multiple OUTPR commands can sometimes be a little tricky. See Section 2.7.4:
Output Controls in the ANSYS Basic Analysis Guide for more information on how to use this command.
2.3.3.9. Extrapolation of Results
Use this load step option to review element integration point results by copying them to the nodes instead of extrapolating them (default when no material nonlinearities are present).
Command(s): ERESX
GUI: Main Menu> Solution> Unabridged Menu> Load Step Opts> Output Ctrls> Integration Pt
2.3.4. Apply the Loads
After you set the desired solution options, you are ready to apply loads to the model.
2.3.4.1. Load Types
All of the following load types are applicable in a static analysis.
2.3.4.1.1. Displacements (UX, UY, UZ, ROTX, ROTY, ROTZ)
These are DOF constraints usually specified at model boundaries to define rigid support points. They can also indicate symmetry boundary conditions and points of known motion. The directions implied by the labels are in the nodal coordinate system.
2.3.4.1.2. Forces (FX, FY, FZ) and Moments (MX, MY, MZ)
These are concentrated loads usually specified on the model exterior. The directions implied by the labels are in the nodal coordinate system.
Section 2.3: Performing a Static Analysis
2.3.4.1.3. Pressures (PRES)
These are surface loads, also usually applied on the model exterior. Positive values of pressure act towards the element face (resulting in a compressive effect).
2.3.4.1.4. Temperatures (TEMP)
These are applied to study the effects of thermal expansion or contraction (that is, thermal stresses). The coefficient of thermal expansion must be defined if thermal strains are to be calculated. You can read in temperatures from a thermal analysis [LDREAD], or you can specify temperatures directly, using the BF family of commands.
2.3.4.1.5. Fluences (FLUE)
These are applied to study the effects of swelling (material enlargement due to neutron bombardment or other causes) or creep. They are used only if you input a swelling or creep equation.
2.3.4.1.6. Gravity, Spinning, Etc.
These are inertia loads that affect the entire structure. Density (or mass in some form) must be defined if inertia effects are to be included.
2.3.4.2. Apply Loads to the Model
Except for inertia loads, which are independent of the model, you can define loads either on the solid model (keypoints, lines, and areas) or on the finite element model (nodes and elements). You can also apply boundary conditions via TABLE type array parameters (see Section 2.3.4.2.1: Applying Loads Using TABLE Type Array Parameters) or as function boundary conditions (see Section 2.6.15: Applying Loads Using Function Boundary Conditions).